As violence escalates between Christians and Muslims in the Central African Republic, the United Nations is warning of a humanitarian disaster of unspeakable proportions.

Although some 6,000 peacekeeping soldiers have been deployed, so far, they have been powerless to stop the religious violence.

On Thursday, they discovered a mass grave in the capital city of Bangui. An eyewitness reported seeing at least a dozen decomposed bodies at a military camp reportedly run by Muslim rebels.

It is still unclear who committed the killings.

"There is an ethno-religious cleansing that is taking place that we must stop at all costs," António Guterres, with the United Nations High Commissioner for Refugees, said.

After enduring months of daily attacks, rape, and torture by Muslim rebels, a Christian militia group called Anti-Balaka is now retaliating with brutal violence, forcing tens of thousands of Muslims to flee.

"What we have seen around the country is enormous forced expulsion of the Muslim population and Anti-Balaka control of the roads and of the towns all across the country," Joanne Mariner, senior crisis response adviser at Amnesty International, said.

About 70 miles from the country's capital city, Father Xavier Fagba has opened the doors of his church to more than 700 Muslims seeking protection from Anti-Balaka forces.

He told BBC News it was time someone stood in the gap for peace and reconciliation.

"Right now there are towns in which you have hundreds of Muslims taking refuge in churches because it's the place they believe are safest from possible Anti-Balaka attacks," Mariner said. "And some of those populations are completely unprotected."

Muslim rebels in the north are reportedly being joined by Islamic fighters from neighboring countries and they are regrouping to launch revenge attacks against Christians.

The developments have forced many Muslims and Christians to pack up their belongings in preparation to flee

Meanwhile, a Boeing 747 carrying critical supplies has landed in the CAR.

"This is a lifeline that will allow us to bring, immediately, emergency food relief to the people who've been affected by the conflict here in the Central African Republic," Alexis Masciarelli, spokesperson for the World Food Program, said.